182 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



C. Development in the Larvae. 



(a.) In the First Larva. 



The stages at which the various phases of the development of the an- 

 tennal gland in the embryo appear are not precisely fixed by time intervals 

 because of seasonal variation in the rate of development. They may be 

 referred to particular conditions of prominent external organs, such as 

 the pigmentation of the eyes or the position of the tip of the telson, with 

 much greater precision, but even this allows of no close comparisons with 

 parallel stages in other genera or families. The stage at which the em- 

 bryo hatches, marking the transition from the passive embryonic to 

 the active larval life, is sharply defined in most free-living Crustacea, 

 and it would seem that this is a time at which comparisons may 

 be made most safely. Moreover, the transition to the predatory life 

 of the larva must have a marked influence upon metabolism, and 

 therefore upon the excretory organ and its functions. It has there- 

 fore seemed well to consider this stage in some detail. 



My material was all killed within twelve hours after hatching, and 

 hence represents the earlier part of the first larval period, the extent of 

 which may be as much as five days (Herrick, '95, p. 172). 



1. General Structure. 



With the hatching of the embryo a considerable alternation occurs in 

 the topography of the antennal gland, but this is due to mechanical 

 causes rather than to growth. The second antenna, which has been 

 folded back against the embryo so that its distal end is posterior to its 

 proximal end, is released in hatching and swings — approximately in a 

 parasagittal plane — through about 135 degrees, to a position in which 

 the distal end is anterior to the proximal. Thus in the basal region of 

 the appendage the dorsal face is shortened and thereby relieved of the 

 stress which has existed at that place during embryonic life. This 

 removal of pressure from the extensions and evaginations of the ecto- 

 dermic sac brings about a change in its form, which passes from the 

 flattened constrained shape to a more rounded one. This permits the 

 separation of the apposing walls and the establishment of a large lumen. 

 Likewise the endsac, which has been compressed between the dorsal and 

 ventral portions of the ectodermic sac, is relieved from this pressure and 

 expands as much as the larger space allows. 



In Figures 32 to 38 (Plate 4) are shown the second, third, fifth, 

 sixth, eighth, eleventh, and twelfth sections respectively from a series 



